get a random number after keyframes done its work / just get a random number every second

let x 
    x =
    Math.floor(Math.random() * 100);
    var r = document.querySelector(':root');
    function set1() {
  // Set the value of variable --blue to another value (in this case "lightblue")
    r.style.setProperty('--at11', x + '%');
}
window.onload = set1;

@keyframe animation-goes{
10%{--sth : var(--at11)
    }
20%{--sth : var(--at11)
    }
30%{--sth : var(--at11)
    }
......
}